1. Technical Field
This application relates to a method for the production of a finely crystalline boehmite and application of said boehmite as flame retardant in plastics.
2. Background Information
From U.S. Pat. No. 4,117,105 the conversion of aluminumtrihydrate to well dispersable boehmite is known. According to one method, aluminumtrihydrate with a BET-surface of 0.2 to 15 m2/g is calcined at temperatures between 200 up to 800° C. for 2 to 17 hours, until the BET-surface has risen to 250 to 800 m2/g. Then a slurry of the calcined alumina is rehydrated at a temperature of 140 to 200° C. for 0.5 to 6 hours in an autoclave.
The Braunauer-Emmet-Teller (BET) method is a method of measuring the amount of gas absorbed on a solid surface. It was first published by its creators in 1938 in a journal; the article was titled “Adsorption of Gases in Multimolecular Layers”.
A flame retardant plastic composition and a method for producing a filler material is known from German Patent No. 19812279 C1. The plastic composition consists of 55 to 75% boehmite in an orthorhombic crystal structure, wherein, depending on temperature control, the BET-surface varies between 14.75 and 17.25 m2/g. As filler material boehmite with a grain diameter of 0.5 up to 3 μm is applied.
From German Patent No. 69231902 T2 a method for the growth of crystals, especially crystals consisting of metal oxides, at increased speed is known. In said method BET-surfaces in a range of 40 m2/g at a crystal size of roughly 50 nm are obtained. Within said method a feed solution with a pH-value of 3 to 11 is provided for precipitation on a seed crystal, which comprises a metal oxide species, which is sufficiently insoluble in the aqueous medium, in order to provide a solid center of growth. The treatment is then carried out under hydrothermal conditions, wherein feed material is added until the end of crystal growth.
In Japanese Patent No. 63265810 A a method is described, in which smooth α-Al2O3-spheres are obtained from an aluminumhydrate, wherein aluminumhydrate is ground in wet condition at a pH-value between 1 and 4. Said α-Al2O3 is obtained subsequently by calcination at 1350 to 1500° C.
From the production of the ion conductor beta-alumina, it is known from German Patent No. 3617115 A1 to mix boehmite with water and to adjust the pH-value of the mixture with acetic acid to a pH-value of 4. The mixture is then ground and sodium oxide and a spinel-stabilizator are mixed into the ground mixture in aqueous solution whereafter the solution is peptised, e.g. by acidifying anew to a pH of 4 with acetic acid and the following production of a gel at raised temperatures (80° C. for 20 minutes). The product can preferably be formed into a self-supporting commodity, consisting of beta-alumina, by isostatic pressing.
A method for the production of quasi-crystalline boehmite from a boehmite-precursor by application of seed crystals in hydrothermal treatment is known from German Patent No. 60015345 T2. Also ground boehmite crystals can be applied as crystals, wherein the hydrothermal reaction is carried out at pH-values equal or smaller than 7. Furthermore the production of microcrystalline boehmite and ceramic bodies is described in German Patent No. 3879584, wherein a precursor of boehmite and boehmite seed crystals are applied under hydrothermal conditions at pH-values of 8 or higher and temperatures of above 130° C. A method for the production of finest-particulate seed crystals is known from U.S. Statutory Invention Registration No. H189. With said seed material alpha alumina is obtained by applying the seed material in a boehmite-gel and transformation at relatively moderate temperatures into finely crystalline alpha alumina. Said material is applied within electrical industrial production or as abrasive.
For the application in flame retardants in plastic compositions a finely crystalline boehmite is required, having a low surface and a low pore volume. The flame retardant should show good and easy miscibility with said plastic compounds while having a high fraction at the total composition and also a high level of inflammability or non-flammability should be obtained. Furthermore the properties of mechanical strength like tensile strength and elongation at break should be on a high level.